a) Field of the Invention
The present invention relates to a microscope apparatus for observing, with high resolving power, a transparent minute object such as bacteria or crystal structure of a metal, and specifically to that using a differential interference microscope or a phase-contrast microscope.
b) Description of the Prior Art
In general, a differential interference microscope is capable of forming an image showing unevenness or inclination of an object to be observed by separating light into ordinary ray and extraordinary ray using a birefringent prism, irradiating the object with these ordinary and extraordinary rays, and causing the rays to interfere as reflected from or transmitted through the object.
These days, attempts are made to measure the phase distribution or minute shape of an object to be observed by applying the conventional technique of two-beam interferometry to differential interference microscopy. In the field of semiconductor manufacturing, specifically, a method for measuring film thickness of a phase shifter of a phase shift mask with high precision is disclosed in Japanese Patent Preliminary Publication No. Hei 6-229724. "Applications of Interferometry and Automated Inspection in Japan, T. YATAGAI, SPIE. Vol. CR46, 1992" and Japanese Patent Preliminary Publication No. Hei 5-232384 show examples of the application of the fringe scan method in interferometry to differential interference microscopy.
When the differential interference microscope is used for the measurement of an object, it is regarded as a double-beam interferometer of the Mach-Zehnder type, as disclosed in Japanese Patent Preliminary Publication No. Hei 6-229724. In this approach information on the object with respect to phase etc. is obtained by determining the difference in optical path length between the ordinary ray and the extraordinary ray based on the supposition that light diffracted about the object has no affect.
In the conventional differential interference microscopy, the separation width between the ordinary ray and the extraordinary ray on the object surface is called "shear", which is set at a value as high as a resolving power of the microscope so that diffracted rays derived from the ordinary ray, and the extraordinary ray respectively, are caused to interfere to form an image. It is known that the shear is an important parameter for determining resolving power of the differential interference microscope and contrast of the image. Therefore, for measuring the phase distribution or minute shape of the object using the differential interference microscope, the shear specific to the differential interference microscope and effect of diffraction about the object should be considered.
However, the above-mentioned "Applications of Interferometry and Automated Inspection in Japan" and Hei 5-232384 suppose that all of the rays diffracted about the object are transmitted to an image surface, disregarding loss of diffracted light dependent on such parameters as numerical aperture of the optical system (response characteristic). Resultantly, accurate phase distribution of the object is difficult to be obtained from the image formed through such a method.
In a conventional phase-contrast microscope, a ring slit and a phase ring are arranged at a pupil position of an illumination system and a pupil position of an objective lens conjugate with the ring slit, respectively, and out of diffracted rays, zero-order rays are changed in phase and intensity to interfere with the remaining diffracted rays so that phase of a specimen (object) is converted into contrast of the image for observation.
As for contrast of phase-contrast image by a phase-contrast microscope, "Some Improvements in the Phase Contrast Microscope, K. YAMAMOTO, A. TAIRA, J. Microscopy, 129 (1983) 49-62", for example, describes a method for improving it. In conventional phase-contrast microscopy, in general, the contrast of the image is improved by arranging diameter of a ring-shaped phase film to be substantially half of a pupil diameter of the objective lens.
Japanese Patent No. Hei 7-60216 discloses a method for determining phase distribution of the object from a phase-contrast image by the phase-contrast microscope. However, as described in "Theoretical Considerations on Phase Contrast Imaging H. OOKI, Optics, Vol.20, No.9, 1991, pp 590-594, imaging by the phase-contrast microscope is disadvantageous in resolving power as compared with an ordinary bright-field microscope because the cutoff frequency is dependent on a ratio of the phase film diameter with the pupil diameter. This disadvantage could be moderated as the ratio approximates 1. However, as described in the above-mentioned document, such solution raises another problem of weakened contrast of the entire image, to obscure the image visually, because contrast of spatial frequency in a low band is weakened.
As a measure for enhancing the contrast without changing the diameter of the phase film, absorption by the phase film would be increased. However, if absorption by the phase film is increased, another problem is raised, as indicated by "Inverse Problem in Microscopy A. TAKAHASHI, I. NEMOTO, Denshi Tsushin Gakkai, Technical Research Report MBE88-58, pp 35-42, 1988, that, for a relatively thick specimen as compared with the wavelength of light, e.g. a biological specimen, an accurate phase-contrast image fails to be reproduced because spatial frequency components not existing in the original specimen (nonlinear terms) have an effect.
As stated above, conventional phase-contrast microscopes do not simultaneously satisfy both the requirements for resolution and image contrast.
Furthermore, according to the method recited in Japanese Patent No. Hei 7-60216, since phase distribution of the object is obtained directly from the phase-contrast image without consideration of the response characteristic of the phase-contrast microscope, the phase distribution obtained varies according to the frequency characteristic of the object. Furthermore, since this method disregards influence of nonlinear components such as halo also, uncertainty of phase distribution attributable to the nonlinear components still remains.
In consideration of the problems in the conventional phase-contrast microscopes, the present inventors have disclosed, in Japanese Patent Preliminary Publication No. Hei 7-225341, a phase-contrast microscope which is constructed so that resolving power is improved without weakening image contrast, and which is capable of accurately converting phase distribution of the object into contrast information.